Parachlorophenylalanine (p-Chlorophenylalanine, PCPA) aka Fenclonine is a synthetic amino acid which acts as a selective and irreversible inhibitor of tryptophan hydroxylase, which is a rate-limiting enzyme in the biosynthesis of serotonin.[1] Fenclonine consequently depletes serotonin in the body and reduces its actions, acting as an indirect serotonin antagonist. It is used in scientific research to investigate the effects of serotonin depletion on behaviour.[2]